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In a wide-ranging speech on a program shrouded in secrecy, he said there must be "near certainty" that no civilians would die in such strikes.

In a renewed push to shut Guantanamo Bay, he also lifted a moratorium on prisoner transfers to Yemen.

Obama also defended the use of drones to kill US citizens who have gone abroad to wage war against America.

"From our use of drones to the detention of terrorist suspects, the decisions we are making will define the type of nation and world that we leave to our children," he said in Thursday's address at National Defense University in Washington.